Earth Systems 3209
Unit: 4
The Forces Within Earth
Reference:
Chapters 4, 15, 16, 19; Appendix A & B
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Unit 4:
Topic 4.3
Properties of Seismic Waves
Focus on . . .
 identify the different seismic waves and describe the
properties of each.
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Seismic Waves
 The source of an earthquake is called the focus and the epicenter
is the point on Earth’s surface directly above the focus.
 Seismic waves originate at the focus and travel outward in all
directions.
These energy waves are classified as;
1) Body Waves:
2) Surface Waves:




Primary Wave
Secondary Wave
Love Wave
Rayleigh Wave
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Text Reference
Pages 445 - 447
Seismic Waves – Body Waves

Originate from the focus and travel in all directions
through the body (Interior) of the Earth.
1)
Primary Wave (P-Wave)

P-waves move by compressing and expanding (push-pull motion) the
material as it travels. Much like sound waves.

These waves can pass through solids, liquids, and gases.

Vibrate in the same direction as wave motion.
Rarefaction

These waves have the greatest velocity
(6 km/sec) and are the first to reach the
seismograph stations.
Compression
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Vibration
Direction
Seismic Waves – Body Waves

Originate from the focus and travel in all directions
through the body (Interior) of the Earth.
2)
Secondary Wave (S-Wave)

S-waves travel through material by shearing it and vibrate in a
direction perpendicular to the wave motion.

Because liquids and gases have no shape, these waves do not
pass through liquids or gases, only trough solids.

These waves are much like the waves on the ocean.

These waves travel through Earth
slower (3.5 km/sec.) and are the
second to reach seismograph stations.
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Vibration
Direction
Seismic Waves – Surface Waves
 Surface waves differ from body waves in that they
do not travel through Earth’s interior, but instead
travel along paths nearly parallel to the surface of
Earth.
 Surface waves behave like S-waves in that they
cause up and down and side to side movement as
they pass, but they travel slower than S-waves.
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Seismic Waves – Surface Waves
1)
Love Wave
 Surface waves that cause horizontal shearing
of the ground. They move in much the same
way as a snake slithering across the ground.
 Vibrate in a perpendicular direction
compared to that of wave motion.
 Surface waves are the most
destructive and cause the
most damage.
Direction of Motion
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Seismic Waves – Surface Waves
2) Rayleigh Wave
 Surface waves that cause both horizontal
(side-to-side) and vertical (up and down)
movement within the ground.
 Vibrate in a rolling motion in the same
direction as wave motion.
 Most of the shaking felt from an
earthquake is due to these waves and
these waves are the most destructive
and cause the most damage.
Direction of Motion
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Seismograph and Seismogram
 P-waves, S-waves, and Surface waves are all
recorded on the seismogram as seen below:
 These paper records are important when seismologist wants to
locate the position of the epicenter of an earthquake.
 Seismologist can determine the difference in arrival times between
the P-wave and the S-wave.
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Seismic Waves - Comparison
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Example 1:
Which description best describes p-waves?
A) they move back and forth (compressional wave) and
travel through any material.
B) they move back and forth (compressional wave) and
move only through solids.
C) they move side to side (transverse wave) and only
travel through liquids.
D) they move side to side (transverse wave) and travel
through any material.
Which seismic wave is the first to reach a seismic station?
A) primary wave
B) love wave
C) secondary wave
D) rayleigh wave
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Example 2:
Body waves consist of ________?
(A) Primary waves
(B) Secondary waves
(C) Love waves
(D) Both Primary and Secondary waves
Which wave is the fastest and moves in a push-pull motion?
(A) L
(B) primary
(C) S
(D) Surface
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Your Turn . . .
Take the time and complete the following questions . . .
(Solutions to follow)
Question:
Contrast the characteristics of
Primary and Secondary waves.
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Solutions . . .
Question:
Answer:
P - wave
Push - pull waves which vibrate in the same direction in which they move.
Fastest earthquake wave and is the first to arrive at seismograph stations.
Pass through all states of matter, solids, liquids, and gases.
S - wave
Shake the particles which cause them to vibrate in a perpendicular
direction to their motion. Slower than P - wave and is the second
earthquake wave to arrive at seismic stations. Pass only through solids.
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Summary . . .
Overview of Points covered:
2 Categories of Seismic Waves:
1) Body Waves
 Primary
 Secondary
2) Surface Waves
 Love
 Rayleigh
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